Wave soldering is a well-known method by which solder connections are formed on a printed circuit board (PCB) by bringing the circuit board in contact with a "wave" of molten solder flowing upward from a nozzle. Wave soldering methods typically apply a relatively thin coating of solder, and therefore find widest use for applications in which a solderable contact or metal lead is to be coated with solder to form a solder fillet or solder connection, respectively. One such example is a circuit component having leads that extend through a circuit board, and to which solder is applied to physically and electrically connect the component to the board. During wave soldering, the circuit board, placed on a pallet, passes through molten solder coming up from the nozzle, such that the ends of the leads projecting through the circuit board are brought in contact with the upward-flowing molten solder. In the process of adhering to the leads, the solder forms solder connections or fillets.
While leaded packaged circuit devices are widely employed in electronic applications, and wave soldering is widely practiced to attach and electrically connect such devices to printed circuit boards, a continuous effort to reduce the size of circuit board assemblies has promoted the use of such advanced packaging technologies as tape automated bonding (TAB), chip on board (COB), flip chips, multichip modules (MCM) and ball grid arrays (BGA). These devices are generally surface-mount technology (SMT) components attached to a circuit board with solder, such as solder bumps that are formed on the devices and then reflowed to solder the devices to appropriate conductor patterns on the board. While highly successful, SMT devices are vulnerable to downstream processes that can raise the temperature of the solder above its melting or solidus temperature, causing the solder to reflow and create shorts and open circuits. Examples of downstream processing that have caused unwanted solder reflow include wave soldering in addition to glue cures, epoxy cures, conformal coating cycles, and solder reflow for other devices elsewhere on the board. Reduced board thicknesses have also increased the occurrence of unwanted solder reflow and may become sufficiently distorted during processing to break solder connections of a SMT component.
As a particular example, a circuit board with SMT components such as flip chips may also have leaded devices that require wave soldering. If 63Sn/37Pb eutectic solder (melting temperature of about 183.degree. C.) is used to form the solder connections for the flip chips, subsequent processes including the wave soldering operation must not raise the flip chip solder connections to a temperature of 183.degree. C. or above. However, wave soldering subsequently performed on the same circuit board with the same or higher-temperature solder used to form the flip chip connections will tend to cause unwanted solder reflow of those connections, during which the solder melts and expands. Because solder joints of a flip chip are typically encased in an organic underfill adhesive, a pressure is generated that can possibly result in adhesive failure of the underfill from the flip chip. Altering the wave soldering parameters, including preheat temperature and belt speed, has not eliminated the occurrence of unwanted solder reflow. There are additional measures that can be taken to reduce the incidence of unwanted solder reflow, such as the use of alternative devices, processes and materials to reduce the temperature to which the SMT components will be exposed, and using solders with higher melting/solidus temperatures to form the SMT solder connections. However, such solutions are often not practical.
Therefore, what is needed is a technique by which unwanted solder reflow of a SMT component can be avoided during a subsequent wave soldering operation. Preferably, such a technique would not complicate processing or compromise solder connections formed by the wave soldering process.